Of the games you just listed, the first two are CPU bound and the last wto are GPU bound... That makes it a bit more difficult
It depends. If your looking for more performance now, get the GPU upgrade. It will be bottlenecked without doubt by the CPU, but you should see an appreciable upgrade in performance. If you ever get around to upgrading the CPU later on, you will get its full performance.
You could also include the CPU cooler on this upgrade so you can overclock, which will mean that the CPU bottleneck will be somewhat alleviated. Dont bother getting a new motherboard.
If your willing to wait longer for even more performance, go the CPU route and get a new GPU later on.